An atomic force microscope is an instrument useful for characterising a surface at atomic resolution. A typical atomic force microscope (AFM) comprises a flexible cantilever probe with a sensing tip at one end and a photosensor to monitor the reflection of light from the free end of the cantilever probe as the probe moves over the surface of a sample.
FIG. 1 shows a prior art atomic force microscope system 10. A light source 12 directs a laser beam through a lens 14. The light passing through the lens 14 is focused on a point on the cantilever 16 near the free end of the cantilever 16. The cantilever 16 has a sensing tip 18 on a face opposite that on which the light beam is focussed. The sensing tip 18 faces a surface of a sample 20.
The sample 20 is mounted on a piezoelectric system 24, which is operable to translate in the X-, Y- and Z-directions. The light beam is reflected from the top of the cantilever 16 onto a position-sensitive photodetector 30. This photodetector 30 can be a bicell or a quadcell design. The light beam is sensed separately by a first photo-element 31 and a second photo-element 32 in a bicell design. In a quadcell design, the photo-element 31 is simply the sum of two adjacent photo-elements at one half side and the photo-element 32 the sum of the other two photo-elements on the other half. The photodetector 30 is mounted on a translational stage 34 that is operable to adjust the photodetector position such that the light beam is equally distributed between the two photo-elements 31, 32. The translational stage 34 is operated manually or by piezoelectric translators.
As the sensing tip 18 follows the topography of the sample 20 surface, the sensing tip moves in a substantially vertical or Z-direction. The light beam is reflected off the back of the cantilever 16 with the amount of light falling on each photo-element 31,32 varying as the cantilever tip moves up and down. Signals from the photo-elements 31, 32 are sent to a differential amplifier 36. The output from the amplifier 36 provides a differential signal to a feedback system 38.
The feedback system 38 generates a correction signal that is applied to the piezoelectric system 24 to translate the sample in the Z-direction in order to maintain a desired spacing or force between the sensing tip 18 and the sample 20 surface. Integration of the correction signal as a function of position across the scanned surface is used to represent the surface topography of the sample 20.
Another optical-based method for AFM sensing as described in U.S. Pat. No. 5,025,658 issued on 25 Jun. 1991 to Digital Instruments, Inc. functions by focussing a laser diode beam on the cantilever in the order of tens of microns from the cantilever tip. The laser light reflected off the cantilever tip enters the laser diode and interferes optically with the light reflected internally from the front face of the laser diode. An interference beam emits from the rear of the laser diode with maximum brightness, total darkness or varying intensity of intermediate brightness. By correlating the change between maximum brightness and total darkness to a quarter wavelength of the laser light, the photodetector output according to the intensity of the light incident on the photodetector corresponds to the vertical movement of the cantilever probe. The photodetector output therefore gives an indication of the topography of the surface on which the cantilever probe traces.
AFM sensing does not necessarily require optical operation. The cantilever itself can be a piezoresistive material and any deflection of the cantilever provides measurable voltage signal changes. Piezoresistive cantilevers are therefore ideal for array cantilever operation because external alignment or calibration is unnecessary. Nevertheless, for common cantilever designs, this type of sensor is Johnson noise limited (as reported by O. Hanson, et al., in Nanotechnology, vol 10, issue 51, 1999) and thus cannot provide the vertical resolution of optical techniques. An alternative is to use capacitive sensing. However, vertical resolution is limited by parasitic capacitances (as reported by S. A. Miller, et al., in Review of Scientific Instruments, vol 68, p 4155, 1997).
Optical lever detection still offers very good force resolution, which is believed to be comparable to interferometry (as reported by Putman, et al., in Journal of Applied Physics, vol 72, issue 6, 1992) while requiring only a relatively simple design.
The assertion of comparable force sensitivity between optical lever and interferometry detection hinges on the presumption that identical optical steepness (i.e. the ratio of optical intensity change with respect to perturbation) exists between them. The creation of light beams with higher optical steepness that affords greater sensitivity has been the subject of some patent applications (eg. U.S. Pat. No. 5,144,833 issued on 8 Sep. 1992 to IBM Corporation). However, there exists a physical limit beyond which the optical steepness cannot be increased further. This is often described to as the “beam waist” effect (as explained by A. E. Siegman, in An Introduction to Lasers and Masers, McGraw-Hill, New York, 1971). In addition, the use of a beam with high optical steepness creates difficulty in aligning the light beam on the cantilever probe and photodetector.
Array cantilevers can operate in tandem to allow higher measurement throughputs and/or measurement of different atomic force interactions (e.g. magnetism, chemical affinity, etc.) via the use of differently doped sensing tips. In the case of array cantilever sensing, optical lever detection is limited by the need to design an array of photodetectors with each photodetector having a provision for position-adjustment to give maximum sensitivity. However, these sensors are expensive to manufacture. More recently, interdigital sensing that function on the principle of diffraction have been reported for array cantilever sensing (eg, by Sulchek, et al., in Applied Physics Letters, vol 78, p 1786, 2001). The diffraction effect is created by micron-sized features that are etched on the cantilevers on sides opposite to the sensing tips. Since a plurality of beams is reflected from each cantilever, the need for position adjustment is obviated. However, the divided intensity reduces optical steepness and in turn reduces the measurement sensitivity. In addition, the manufacture of micron size features on the cantilever is expensive. More significantly, poor production of these micron-sized features affects the quality of the diffraction pattern and hence influences measurement accuracy.
Another issue in the design of the optical cantilever sensing is noise. The presence of noise can easily eliminate whatever gains are derived from designs with improved sensitivity. One convenient method of introducing noise rejection is via light modulation. The signals produced on the photodetectors can then be electronically processed to reduce the effect of noise. This has been the subject of some patent applications (eg. U.S. Pat. No. 5,357,105 issued on 18 Oct. 1994 to Quesant Instrument Corporation and U.S. Pat. No. 5,567,872 issued on 22 Oct. 1996 to Canon Kabushiki Kaisha).
Yet another issue in AFM is the mode of deflection of the cantilever. Depending on the scan direction or surface characteristics, the cantilever can exhibit deflection in the bending and/or torsion modes (as reported by Meyer and Amer, in Applied Physics Letters, vol 57, p 2089, 1990). Independent measurements of bending or torsion deflection would permit improved characterization of a sample surface.